As demand for ever-shrinking semiconductor device features continues to increase so too will the demand for improved optical meteorological techniques. Optical metrology techniques may include critical dimension (CD) metrology, thin film thickness and composition metrology, and overlay metrology. These optical metrology techniques may be carried out utilizing a variety of optical architectures including scatterometry-based optical systems, reflectometry-based optical systems, ellipsometry-based optical systems, and spectrometry-based optical systems.
Typically, optical metrology systems utilize light sources operating in a constant-current or in a constant-light-output mode in order to ensure optical stability of the system as well as keeping noise levels within tolerated limits.
In optical metrology settings where coherent light sources are implemented, the production of coherent artifacts, such as interference fringes resulting from duplicate images (i.e., “ghosts) and speckle, are significant concerns in the operation of the given optical metrology tool. Due to the large coherence length of laser-based illumination sources, minimizing the impact of coherent artifacts can be challenging. Coherent artifacts manifest in optical metrology settings where the coherence length, often 100 m or more, of the utilized illumination is larger than the distance between light reflecting surfaces of the metrology tool. Such reflecting surfaces may include lenses, beam splitters, optical fibers and the like. In this scenario, a primary beam will constructively interfere with illumination from a parasitic beam, leading to the production of ghost induced interference fringes. The interference contributions may grow to such a degree that they possess intensity values on the same order of magnitude of the primary beam, thereby severely hampering the usability of the given optical metrology tool.
In addition, some metrology applications require time-sequencing intensity control of multiple illumination sources emitting different wavelengths of light. The prior art accomplishes time-sequencing intensity control utilizing various optical-mechanical and electro-optic device such as shutters, acousto-optic devices, Pocket's cells, and the like. The prior art uses of such devices to control the time-sequencing of multiple illumination sources may lead to reduced stability and repeatability.
Therefore, it would be advantageous to cure the shortfalls of the prior art and provide a system and method for mitigating the effects of coherence artifacts and additional noise sources in an optical metrology setting. In addition, it would be advantageous to produce a system and method providing an efficient means for time-sequencing of multi-wavelength illumination source outputs for multi-wavelength optical metrology applications.